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“They were very happy. It was a relief to see the end of their suffering,” he said.

“They had a cup of coffee in the hall, it went well and a rich conversation. The the separation from their parents and brother was very serene and beautiful. At the last there was a little wave of their hands and then they were gone.”

Under Belgian law euthanasia is allowed if the person wishing to end his life is able to make their wishes clear and a doctor judges that he is suffering unbearable pain.

The case is unusual because neither of the men was terminally ill nor suffering physical pain.

Just days after the twins were killed by doctors, Belgium's ruling Socialists tabled a new legal amendment that will allow the euthanasia of children and Alzheimer's sufferers.

The major and controversial change will allow minors and people suffering from dementia to seek permission to die.

"The idea is to update the law to take better account of dramatic situations and extremely harrowing cases we must find a response to," said Thierry Giet, the Socialist leader.

If passed later this year, the new law will allow euthanasia to be "extended to minors if they are capable of discernment or affected by an incurable illness or suffering that we cannot alleviate".

In 2002, Belgium was the second country in the world after the Netherlands to legalise euthanasia in but it currently only applies to people over the age of 18.

Some 1,133 cases of euthanasia - mostly for terminal cancer - were recorded in 2011, according to the last official figures.

In 2011, it emerged that people killed by euthanasia in Belgium are having their organs harvested for transplant surgery.

Last December, the Belgium-based European Institute of Bioethics published a report raising concerns over "the absence of any effective control" over euthanasia and "ever?widening interpretation" of the law.

The institute noted that over 10 years and 5,500 euthanasia cases not one had ever been referred to the police for investigation.